Exhaust hoods are used to remove air contaminants close to the source of generation located in a conditioned space. For example, one type of exhaust hoods, kitchen range hoods, creates suction zones directly above ranges, fryers, or other sources of air contamination. Exhaust hoods tend to waste energy because they must draw some air out of a conditioned space in order to insure that all the contaminants are removed. As a result, a perennial problem with exhaust hoods is minimizing the amount of conditioned air required to achieve total capture and containment of the contaminant stream.
Referring to FIG. 1, a typical prior art exhaust hood 90 is located over a range 15. The exhaust hood 90 has a recess 55 with at least one vent 65 (covered by a filter 60) and an exhaust duct 30 leading to an exhaust system (not shown) that draws off contaminated air 45. The vent 65 is an opening in a barrier 35 defining a plenum 37 and a wall of the canopy recess 55. The exhaust system usually consists of external ductwork and one or more fans that pull air and contaminants out of a building and discharge them to a treatment facility or into the atmosphere. The recess 55 of the exhaust hood 90 plays an important role in capturing the contaminant because heat, as well as particulate and vapor contamination, are usually produced by the contaminant-producing processes. The heat causes its own thermal convection-driven flow or plume 10 which must be captured by the hood within its recess 55 while the contaminant is steadily drawn out of the hood. The recess creates a buffer zone to help insure that transient, or fluctuating, surges in the convection plume do not escape the steady exhaust flow through the vent. The convection-driven flow or plume 10 may form a vortical flow pattern 20 due to its momentum and confinement in the hood recess. The Coanda effect causes the thermal plume 10 to cling to the back wall. The exhaust rate in all practical applications is such that room air 5 is drawn off along with the contaminants.
Referring now also to FIG. 2, exhaust hoods 90, such as illustrated in FIG. 1, vary in length and can be manufactured to be very long as illustrated in FIG. 2. Here multiple vents 65 can be seen from a straight-on view from the vantage of a worker 80. The length can present a problem because the perimeter along which capture and containment must be achieved is longer near the ends than in the middle. In the middle, there is only one perimeter, the one along the forward edge indicated at 70 in FIG. 1. At the ends, this perimeter includes the side edge as well which is indicated at 75 in FIG. 1. The additional perimeter length that must be accommodated at the ends may be called an “end effect.” In other words, the hood cannot be approximated as a two-dimensional configuration because of its finite length. As a result of the increased perimeter at the ends, more air must be exhausted in the vicinity of the ends of the hood than in the middle because the perimeter at the ends consists of both the forward edge 70 of the hood adjacent the worker and end edges 75, which are perpendicular to the forward edge 70.
If the minimum exhaust rate for the entire hood is to be achieved, then less air should be exhausted near the middle section than near the ends. Otherwise, an excess rate of air exhaust will occur near the middle section to insure the rate at the ends is sufficient. Thus, as a result of the end effects and the requirement of full capture and containment, more air must be drawn through the middle section than necessary. In addition, a higher volume of effluent may be generated at some parts of a hood than at others. This variability leads to the same result: some parts of the hood may require a greater exhaust rate than others.
Referring to FIG. 3, a similar problem occurs when multiple hoods are connected to a single exhaust system. For example, the hoods may be connected to a common exhaust duct 191. Each hood must be balanced against the others so that each exhausts at the minimum rate that ensures full capture and containment of the contaminants. Again, ducts carrying grease aerosol should not have dampers because of the hazard caused by grease precipitation.
It is known in the prior art to make fixed modifications to the flow requirements of a long hood to address the end effects.